Seized drop keel bolt

Does anybody know what size the keel bolt is please.
It is seized solid to the keel and rotates when the keel is lifted or lowered.  This has caused a leak between the bolt and the housing.
The plan is to free this off and replace the bolt with possibly stainless steel.
The first job is to drill and tap a hole in the keel box directly above the bolt to get some penetration oil in there.  Making sure we fit a plug before going back on the water!
Any information would be appreciated.
Thanks

I can get you the size i used when i replaced it with a stainless bolt, it will be a metric size so slightly different from the original but it fits the keel and the box fine. I'll try to remember when im down the yard next. I remember the nut was 30mm but cant remember the bolt diameter off the top of my head.

I replaced mine with a 20mm Bolt. I would assume it was the closest imperial size to that.

Hi
Thanks for this information.  I think it will be a winter job.  we have made a temp repair so the leak has stopped and we dont want to miss any sailing.